( echnical ea Ufe )

ByIan Braithwaite, G4COL ".

ponent. A broadband active loop antennais still on my list of things to try.

My first homebrew active antenna wasa dipole, and was quite successful. Themain thing it taught me was that it's not agood idea to have too much gain. It isnatural to conclude that, as a short an­tenna picks up a smaller signal than aresonant dipole, the gain must be madeup in the amplifier. Being a broadbanddevice, the amplifier is subjected to theentire HF radio spectrum including pow­erful broadcast transmitters. What tends

Power feed unit

To Rxr-----O + 12V

to happen in prac­tice is that it dis­torts, generatingintermodulationproducts. Theseappear to the re­ceiver as additionalsignals and,though giving theimpression of a'lively' receivingsystem, are en­tirely unwanted. Anattenuator be­tween the activeantenna and re­ceiver is of no useat all, if the distor­tion has alreadyhappened in the ac­tive antenna.

It occurred to meto try a single wire monopole, which madefor a simpler amplifier. This worked andhas been in use ever since.

CIRCUIT DESCRIPTIONTHE AMPLIFIER, shown in Fig 1, is asource-follower circuit designed aroundTr1, a J310 FET (field-effect transistor).This has to present a high impedance tothe small monopole, otherwise signalvoltage is lost, and then deliver the signalto the receiver input, commonly a 50Qimpedance.

* See text

Coax cableA A

R3100R

Antenna unit

TRIJ310t;~f:~na

element

>-I .......~-+-wCl

lOOp

Rl1M

©RSGB RC3045

HIS activeantenna hasbeen doing

its job in my loft forwell over 10 years.I've seen a numberof designs over theyears, and this isthe simplest, deriv­ing from experi­ment. When I re­cently saw a highlysophisticated-Iook­ing commercialunit, I felt it wastime to 'go public'.

Active antennasrely on a combina­tion of an antennaelement (such as a

dipole, monopole, The active antenna circuitry built into a diecast box.or loop) and an am-plifier, which is the 'active' part. The an­tenna element is non-resonant, and tendsto be physically small. They have broadoperating bandwidths, so don't need tobe tuned. In comparison, a resonant an­tenna would need tuner adjustments tocoverthe whole HF and lower VHF spec­trum. So, the attraction of active anten­nas is convenience.

It is only fair to point out that somepeople dislike them, and there are pitfalls,which I shall point out. If you want a reallyexcellent receiving antenna for all the HFamateur and broadcast bands, and havemasses of space, why not put up a Bev­erage or rhombic antenna? If, as in mycase, that's out of the question, thenconsider an active antenna and, betterstill, try building your own! This one canbe put together in a few hours and covers160 to 4 metres.

DESIGN CONSIDERATIONSTHE CHOICE of a small antenna element(less than a tenth of a wavelength or so)is between the dipole and monopole,which respond to the electric field com­ponent of the radio wave; and the loop,which responds to the magnetic field com-

* 28 Oxford Avenue, St Albans, Herts ALI 5NS.E-mail: ianb@g4col.demon.co.uk Fig 1: Circuit diagram of the antenna and power feed units.

52 RatlCom + October 2001

-9dB

r:r-- - ............r-;r--. -

-19dB

Fig 2: Frequency response of the active antenna.

CONSTRUCTIONTRANSFORMER T1 requires some carein construction, and is described in somedetail, starting with the quadrifilar wireitself. This would probably be a labour­intensive and expensive item to producecommercially, and is where the ama­teur's craft skills come into their own.

Take four strands of 0.2mm diameter(35/36 SWG) enamelled copper wire,length approximately 300mm for eachstrand. Placing the wires side-by-side,clamp one end and, pulling the wirestaut, fix the free end in the chuck of ahand drill. Turn the drill to twist thestrands together. There is no need totwist too tightly, a few twists per centi­metre being adequate.

The core should be a high-permeabil­ity (greater than 100) ferrite toroid, 10 to15mm in diameter. The purpose of thecore is to produce a sufficiently highinductance to avoid gain roll-off at lowfrequencies, and given a high enoughpermeability, a wide variety of types, stillto be found at rallies, should be suitable.If you are buying new, Table 1 shows thetypes that should be suitable. Suppliercontact details are given at the end of thearticle. Between them, they should beable to source all items needed for con­struction.

Wind seven or eight turns of thequadrifilar wire on the core. (Each timethe wire passes through the core countsas one turn.) The photograph showshow this has been done on a T37 -61core. To secure the winding, the corehas been dipped in polyurethane varnishand left to dry.

The individual wires need to be sepa­rated and the windings identified. Eachwire end should be stripped of its insula-

Stop 100MHz

© RSGB RC3046

REF -14dB

tion. An easy way to do this is to hold thewire end in a blob of solder on the end ofa soldering iron for a few seconds. Makesure you do this in a well-ventilated areaand avoid inhaling the fumes or gettingthem in your eyes.

The ends of each winding can then beidentified with a multimeter or continuitytester. I found it useful to markthewindingswith short strips of insulation strippedfrom ribbon cable, and slid overthe wiresas shown in the same photograph. If youdo this with three windings, the fourth canbe left plain.

Naming the windings arbitrarily 1 to 4,take the end of winding 1, and twisttogether with the start of winding 2. Theend of 2 is then twisted with the start of3, and so on. Twist fairly close to thetoroid, and make electrical connectionusing the soldering iron, as describedabove, observing the precautions. Thetransformer is now complete. Check forelectrical continuity through the wholetransformer by measuring across theun-paired wires.

Once the transformer is done, the restof the construction is straightforward.Start with R2 as 47Q or 68Q. It may needto be changed on test. The photographon p 52 shows my loft unit built into adiecast box, with a couple of solder tagsfor earthing to the box. Alternatively, thecircuit can be built above a small pieceof plain copper-clad board, which canthen be fitted inside a weatherproof en­closure if outdoor mounting is required.The enclosure itself can be plastic - it isan antenna after all!

Make sure you select the correct tap ontransformer T1 for the output, and takecare to prevent the unused taps fromshorting to any other part of the circuit.

dB MAG IdBCH1521

START 9kHz

current is beingdrawn, and pro­tects against inad­vertent supply po­larity reversal.Shown on the cir­cuit diagram is apower feed for a re­ceiver. This is forthe case where thereceiver and an­tenna can sharethe same powersupply. You maychoose to omit it.

The frequencyresponse is shownin Fig 2, and isnominally flat towithin 1dB to60MHz and within2dB to 100MHz.

Detail of the toroidal transformer, n. Notice theplacement of the individual turns, and theiridentification with coloured sleeving from ribbon(rainbow) cable.

Table 1: Suitable toroidal cores and their suppliers.

The FET has an output impedance inthe region of 50 to 100Q, which meansthat, if the FET source fed the receiver50Q input, more than half the signal volt­age would appear across the FET, andless than half would be delivered to thereceiver. That's where the transformerT1, in the source circuit, comes in. I useda quadrifilarwinding to give a 4:1voltagestep down ratio. This gives the sourcefollower an overall gain of almost 1/

5in

voltage (-14dB when the ratio of gatevoltage to output voltage is expressed indecibels).

The benefit of doing this is that the FEThas much less work to do. The action ofthe transformer makes the impedancepresented to the FET source bigger by afactor of 42 = 16 times, which is 800Q.The result is improved linearity. Loca­tions differ, but I have never known theactive antenna produce unwanted sig­nals. You may be concerned that this lowgain would produce a rather 'deaf' receiv­ing system but, from experience, com­paring it to a transmitting dipole and tuner,you won't miss much, if anything. Theinternally-generated noise is very low,and the background noise in most of theHF spectrum is high.

Power to the active antenna is fed viathe coaxial cable, and the supply isinjected via choke RFC1 housed in thepower-feed unit near the receiver. R4 isincluded to limit the current in the eventof an accidental short-circuit. An LED inseries with the supply indicates that

RatlCom + October 2001 53

Fig 3: Circuit diagram of the antenna and power feed units for a positive - earth version.

To Rx

12V+

R3100R

.----, ....- -,---0-12V

+From mains

adaptor

Power feed unit

omitted. This is shown in Fig 3, below.However, note that, while this is fine on itsown, it must not be connected to a re­ceiver with a negative earth, because thisshorts out the supply. •

* See text

Coax cableA A

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TRlJ310t;~f;~n.

element

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100p

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The power feed unit.

from a flat gain with frequency.

POSITIVE - EARTH VERSIONWITH A NEGATIVE supply and positiveearth, a couple of components can be

TESTING ANDCOMMISSIONINGCHECK CAREFULLY forwiring errors. Forbench testing, the power feed and antennaunits can be linked with a short coaxialcable. Having ensured that its voltage andpolarity are correct, connect the powersupply and check that the LED is lit. Meas­ure the voltage across R2 and divide this byits resistance to find the current, or meas­ure the supply current directly. This shouldbe in the region of 10 to 20mA. I selected R2for a current of around 15mA.

Ifthe power feed output is now connectedto a receiver, a small amount of additionalhiss should be heard. Nothing should beheard until a short wire (1 metre or less) isplaced on the antenna input. Signals shouldbe heard on the HF bands, given suitablepropagation conditions, or perhaps televi­sion or PC monitor timebase harmonics.

The antenna unit should be installed ashigh and as far away from local sources ofinterference as practicable. Mine is at theapex of the loft, with an antenna wire ofaround 1 metre length, suspended from ahook in the highest beam. Avoid the temp­tation to increase the wire length exces­sively in order to increase the signal. Thisbrings the risk of distortion, and departure

THE POWER FEEDAS THE PHOTOGRAPH (right) illustrates,I built my power feed unit in a small plasticbox. The choke isa single winding of around20 turns on another high permeability toroid,which can be the same type as that usedforthe transformer. A metal enclosure wouldmake sense for the power feed unit, sinceit will be near the receiver and possibly alsodomestic interference sources. If using aplastic housing, link the coaxial socketswith coaxial cable: I used some RG178.Keep the braid 'tails' short to avoid un­wanted pickup.

54 RatlCom + October 2001